Alkaline Hydrolysis Kinetics and Carbohydrate Retention in Continuous Digesters
Controlling effective alkali profiles and temperature in continuous digesters maximizes xylan retention, stabilizing pulp yield and securing compliance files.
Xylan Redeposition
Polymer Sorption
Dissolved hemicellulose polymers precipitate back onto insoluble cellulose microfibrils during the late cooking stages of alkaline chemical pulping. Triggered by falling alkali concentrations in the cook liquor, xylan redeposition forms a modified carbohydrate surface coating across wood pulp fibres. The returned hemicellulose increases final unbleached pulp yield and alters surface bonding properties.
Sorption Dynamic
As effective alkali levels fall below critical thresholds toward the end of the kraft digestion, dissolved xylan molecules lose their solubilising uronic acid side groups. Reduced electrostatic repulsion and lower liquor pH allow deacetylated xylan chains to align with and crystallise onto exposed cellulose surfaces. Redeposited xylan improves pulp refinability during papermaking by promoting fibre swelling and increasing internal fibrillation during mechanical beating.
Paper made from these enriched fibres demonstrates superior tensile strength and sheet density due to improved inter-fibre hydrogen bonding. Mill engineers lower end-of-cook temperatures and manage residual alkali charges to maximize redeposition rates without causing premature cellulose degradation. Careful monitoring secures pulp yield gains without destabilizing liquor circulation systems.
Bleaching Impediment
Precipitated hemicelluloses trap residual lignin fragments against the outer fibre wall, resisting chemical extraction during subsequent bleaching sequences. Mills producing high-brightness white grades must consume additional chlorine dioxide or apply xylanase enzyme treatments to strip the redeposited layers and release trapped chromophores.